Field of the Invention
The present invention relates to dentistry and more particularly to dental burs in restorative dentistry.
Description of the Related Art
Structural loss in a tooth from dental caries, external trauma, or during tooth preparation can be addressed with dental restoration. Computer-aided design (CAD) and computer-aided manufacturing (CAM) dentistry is an area of dentistry utilizing CAD/CAM technologies to produce different types of dental restorations, including crowns, inlays and onlays, veneers, fixed bridge abutments, and dental implant restorations. The concept of chair side CAD/CAM restoration differs from conventional dentistry in that the prosthesis is typically luted or bonded in place the same day, whereas conventional dental prosthesis of larger size such as crowns have temporaries placed for several weeks while a dental laboratory produces the restoration offsite.
Typically CAD/CAM restorations are milled from solid blocks of composite resin or porcelain matching the shade of the restored tooth. Before the restoration is milled, a tooth must be first prepared for the replacement with a restorative material. The process of preparation usually involves cutting the tooth to make space for the planned restorative material and to remove any dental decay or portion of the tooth that is structurally unsound. The tooth must be reduced enough to allow the ceramic material to be thick enough when chewed on by the teeth in the opposing arch to match a minimum depth as recommended by a manufacturer. After the problem is removed from the disease tooth, proper medications and base materials are placed and shaped and an impression is taken of the resulting preparation. The impression can be made with an elastic material or with a 3-D image. When utilizing CAD-CAM technologies, the data that forms the images is used by computer software to create a virtual replacement of the natural tooth morphology. The software then sends the virtual data to a milling chamber where the dental restoration is carved out of the restorative material. The dental restoration can be further adjusted in the patient's mouth, polished, and then bonded in place.
Of note, the software of the CAD-CAM equipment assumes that a dentist removes enough tooth structure to meet the recommendations of a manufacturer for the selected type of restoration and restorative ceramics. Since the virtual ceramic design must fit between the top of the preparation and the opposing teeth, the dentist must check the depth of the ceramic during the virtual design process or use cutting tools, which can give exact, preselected depth cuts. When the dentist does not have a measured cutting tool, he or she must check the depth after the impression and the design has been completed. If depth is not adequate, the dentist must return to the mouth and take more tooth off.
Also of note, in the method advocated for tooth preparation, the width and depth of an initial central-fossa cut during tooth preparation depends on the material to be used. Most ceramic manufacturers recommend a minimum cut of two millimeter in width for all ceramics. The recommended depth of a cut varies on the material used. For instance, if a feldspathic ceramic block (also known as feldspathic porcelain) is used, the manufacturers recommend a cut of two millimeter in depth; for lithium disilicate blocks a one millimeter depth for inlays is recommended, but a one and half millimeter depth cut for restorative work elsewhere; for leucite reinforced blocks a one and half millimeter depth cut is recommended; and, for composite blocks, such as a composite of ceramic particles and BISGMA polymer, a one millimeter depth cut is recommended. Of further note, the depth of a cut is measured from the central-fossa (the “valley” between the lingual cusp incline and the buccal cusp incline) to the surface of the remaining dentin. The central-fossa is a line that extends from the end of the triangular ridge area of a tooth. It goes up and down (gingivo-occlusally) in vertical height as the topography of a tooth changes. Of even further note, because CAD/CAM software re-establishes this central-fossa line which goes up and down following the cusp inclines of a tooth, if the cut is not appropriately sized vertically, when the image is taken, the resulting milled product can be too thin in some areas and thicker in others, thus the ceramic of the restoration would not have the required physical properties, which could lead to the fracturing of the ceramic requiring that the entire procedure be redone.